I understand the rules for contracting vowels, but I am not sure when to do so. I have inferred that with regular verbs, you contract when there is a vowel at the end of the verb stem, but not when that vowel is part of the first syllable, as one would find in luw, for instance, or plew. Is that correct?

In regards to Attic Greek, a Contract Verb is any verb whose present stem ends in -a, -e, or -o [where o is omicron and w is omega]. Therefore, luw (in beta code lu/w) has a present stem which ends in -u so it is not a contract verb. <br /><br />

What about πλέω? It seems to be a contract as in πλεῖ πλεῖτε in present , but then 1st aorist is ἔπλευσα (<πλευ-). Is the imperfect ἔπλουν, ἔπλεις, ἔπλει? Also imperfect in epic dilalect is πλέον. Is it just because the "contract version" had not appeared yet?

(although, as usual this is followed by a list of exceptions. The most important verbs which do behave this way are: πλέω sail, θέω run, πνέω breathe, δέω need and δέομαι want.)

I believe the reason these verbs don't contract is because there was originally a sound between the stem and the ending which dropped out at that point (usual culprits are sigma and digamma, but I don't know for certain in this case).

I'm a little confused with Smyth's explanation here -- isn't it that in two-syllable εω verbs, the ε only contracts with ε and ει, rather than saying it doesn't contract with ο and ω. The two formulations give different answers to what happens with ε + η. But I know δέῃ is the form of the present subjunctive (Smyth also lists the form) and for other verbs the uncontracted form is the one that turns up in google searches. What's the situation here?

modus.irrealis wrote:[...] isn't it that in two-syllable εω verbs, the ε only contracts with ε and ει, rather than saying it doesn't contract with ο and ω. [...]

Yes, indeed, this is the correct descripiton of the case. ε+ε= ει, ε+ει=ει. This is the rule with the maximal descriptive simplicity. In all other environments, ε remains uncontracted: πλέητε, θέοιμεν, ἐπλέομεν, δεώμεθα, δεομένη, δέον...